The global scourge of polio has been virtually eradicated, reaching historically low numbers this year. But pockets of the disease remain in South Asia and Africa because of the refusal of some parents to immunize their children.

International public health officials counted 177 polio cases worldwide for the first ten months of this year. That’s a drop from just over 500 cases in 2011. Public health officials credit the drop to successful immunization campaigns against the illness, which attacks the nervous system and can cause partial or total paralysis. The malady has disappeared from most countries where it was once epidemic. For example, in India, there have been no cases of polio reported in two years.

But in neighboring Pakistan and Afghanistan, and in Nigeria, West Africa, reservoirs of the viral illness remain. Experts say that is due to the refusal of many parents to vaccinate their children against the infection.

Anita Zaidi is head of pediatrics at Aga Khan University in Karachi, Pakistan. She says intensive public health efforts are underway to vaccinate children against polio. But she says the gains are fragile in some areas and among some ethnic groups, particularly among the country's Pashtun community.

Zaidi says 74 percent of Pashtun children go unvaccinated because many parents believe the immunization is harmful.

“They believe that it can cause sterility in their children or that it’s a conspiracy to sterilize Muslim populations so that their population growth falls, or they believe in the ..value of a vaccine so they think it’s not harmful but it won’t do anything so why take it," she said.

Polio is acquired through contact with feces-contaminated water. Often, Zaidi says, infected children don’t develop symptoms right away so they are unaware that they are exposing other children to the disease. This scenario is common in very dense urban slums, where the availability of clean water is low.

But polio is easily preventable with a series of oral vaccines beginning in infancy.

Zaidi says the key to a successful vaccination campaign in these pockets of polio infection is to engage members of the community to help.

“So that is you have a refusal, [so] you get somebody from that community that you’ve built up trust with, that the community has built up trust with, and you get them to talk to the family," she said.

Pediatrician Anita Zaidi presented a progress report on polio eradication efforts at a meeting this week of the American Society of Tropical Medicine and Hygiene in Atlanta, Georgia.